1. Field of the Invention
The present invention relates generally to waveguide structures and more particularly to waveguide filters.
2. Description of the Related Art
Of the various types of electromagnetic transmission structures, closed metal cylinders are often the transmission line of choice when low loss and high power are critical parameters. These closed cylinders are called waveguides with each waveguide type having a characteristic cross-sectional configuration (e.g., rectangular and circular). Waveguides generally operate as though they were high pass filters (i.e., they have cutoff frequencies f.sub.c and electromagnetic signals at frequencies below f.sub.c are not propagated).
The conducting walls of rectangular waveguides establish boundary conditions that permit the presence of distinct electromagnetic field configurations. These configurations are known as waveguide transmission modes and they are dependent on waveguide characteristics (e.g., cross-sectional dimensions and waveguide dielectric properties). By convention, the wide and narrow walls of rectangular waveguides (and their dimensions) are respectively represented by the letters a and b. In rectangular waveguides, the most common modes are transverse electric modes (TE.sub.mn) and transverse magnetic modes (TM.sub.mn) in which the subscripts m and n respectively represent the number of half-cycles of field variations along the a and b waveguide walls. Each mode is associated with a respective cutoff frequency and the mode with the lowest cutoff frequency is referred to as the fundamental mode with other modes referred to as higher-order modes.
The dominant mode in rectangular waveguides is the fundamental TE.sub.10 mode whose electric field lines 22 and magnetic field lines 24 are shown in the waveguide 20 of FIG. 1A. Note that the electric field vectors 22 define a single half-cycle field variation along the a dimension of the waveguide 20 and the vector magnitudes diminish to zero at the conducting side walls b. There are no field variations along the b dimension. The cutoff frequency and cutoff wavelength (.lambda..sub.c) in the TE.sub.10 mode are given by ##EQU1## in which .mu. and .epsilon. are respectively permeability and permittivity.
FIG. 1B shows the electric field lines 26 and magnetic field lines 28 of the TE.sub.20 higher-order mode in the waveguide 20. Note that the electric field vectors 26 define two half-cycle field variations along the waveguide's a dimension. The cutoff frequency and cutoff wavelength in the TE.sub.20 mode are given by ##EQU2## Because of its electric field pattern's symmetry with respect to a/2, the TE.sub.10 mode is called a symmetric mode. In contrast, the TE.sub.20 mode is considered to be an asymmetric mode.
In an electronic system, nonlinear transmission processes (e.g., nonlinear amplification) are the typical generators of harmonics (signals having frequencies which are integral multiples of a fundamental signal's frequency). In contrast, waveguide width and height discontinuities (e.g., H-plane and E-plane bends, screws, probes, misaligned flanges and wall dents) are the prime generators of higher-order modes. Although symmetric discontinuities (e.g., a symmetric inductive iris) generally generate symmetric modes, asymmetric discontinuities (e.g., a misaligned waveguide junction) can generate symmetric and asymmetric modes.
In a waveguide system that is fed by a fundamental mode, any discontinuity (e.g., an iris or a probe) establishes a complex set of local boundary conditions which can only be satisfied by the presence of a plurality of higher-order modes that are coupled to the fundamental mode. If the transmission frequency is in the waveguide's monomode region (i.e., the frequency region between the fundamental cutoff frequency and the nearest higher-order mode's cutoff frequency), these higher-order modes will be evanescent (i.e., they decay exponentially in the vicinity of the discontinuity). In this situation, the higher-order modes are only required locally to satisfy local boundary conditions and do not propagate through the system. Although a portion of the fundamental mode's energy was locally converted to the higher-order modes, this energy portion is converted back to the fundamental mode as the higher-order modes decay.
Conversely, if the operating frequency is above the waveguide's monomode region or an integral mulitple of the transmit frequency, one or more higher-order modes decouple from the fundamental mode and each of them independently propagates through the waveguide system with different phase velocities (i.e., with different guide wavelengths). The waveguide system is then said to be overmoded and the energy portion that was converted from the fundamental mode is not returned but is independently carried by the higher-order modes. At a second discontinuity, these independently propagating modes can couple again and effect a further exchange of energy between modes.
Systems which contain both nonlinear processes and waveguide discontinuities must therefore contend with the presence of harmonics and of propagating higher-order modes. Such a nonlinear, overmoded situation typically degrades the performance of system devices which are designed to process a fundamental mode but not higher-order modes that are each propagating with different mode patterns and guide wavelengths. In addition, the harmonics may degrade system performance by appearing in other operational frequency bands.
Preferably, the higher-order propagating modes are reduced while the fundamental mode is transmitted. Some waveguide filters (e.g., waffle-iron filters) have the capability of rejecting different higher-order modes but they typically have small vertical gap dimensions which may cause multipacting or arcing in high-power systems. Other waveguide filters (e.g., corrugated filters) can process high power and can be configured to reject a specific higher-order mode. Because their filtering characteristics are a function of a signal's guide wavelength, however, their processing of other modes (such as the fundamental mode) may be unsatisfactory (e.g., see Matthaei, George L., et al., Microwave Filters, Impedance Matching Networks and Coupling Structures, Artech House, 1993, Norwood, Mass., section 7.0.4).